XL stands for "Extensible Language". It's a programming language designed to address the challenges faced by programmers today. It addresses these challenges not by adding language features, but by making it easy and safe for any programmer to add features themselves.

XL has no primitive types nor keywords. All useful operators and data types, like integers or addition, are defined in the standard library

XL features programmer-reconfigurable syntax and semantics. Compiler plug-ins can be used to add new features to the language. A base set of plug-ins implements a relatively standard imperative language. Programmers can write their own plug-ins to implement application-specific notations which can then be used similarly to built-in language features.